High-strength cold-rolled steel sheet and method for manufacturing the same

ABSTRACT

A high-strength cold-rolled steel sheet excellent in terms of elongation (EL) and hole expansion ratio (A) having a low yield ratio (YR), as well as a method for manufacturing the steel sheet. The steel sheet has a chemical composition containing, by mass %, C: 0.15% to 0.25%, Si: 1.0% to 2.0%, Mn: 1.8% to 2.5%, P: 0.10% or less, S: 0.010% or less, Al: 0.10% or less, N: 0.010% or less, and the balance being Fe and inevitable impurities, and a multi-phase microstructure including ferrite having an average crystal grain diameter of 5 μm or less in an amount of 30% to 55% in terms of volume fraction, retained austenite having an average crystal grain diameter of 2 μm or less in an amount of 5% to 15% in terms of volume fraction, and tempered martensite having an average crystal grain diameter of 2 μm or less in an amount of 30% to 60% in terms of volume fraction, in which the number of grains of the retained austenite existing in an area of 1000 μm 2  is 10 or more.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2015/006347, filedDec. 21, 2015, which claims priority to Japanese Patent Application No.2015-038506, filed Feb. 27, 2015, the disclosures of these applicationsbeing incorporated herein by reference in their entireties for allpurposes.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a high-strength cold-rolled steel sheethaving a high elongation (EL), a high hole expansion ratio (λ), and alow yield ratio (YR) and a method for manufacturing the steel sheet, andin particular, to a high-strength cold-rolled steel sheet which can beused for structural parts of, for example, an automobile.

BACKGROUND OF THE INVENTION

In a situation where an increase in fuel efficiency through the weightreduction of an automobile body is an important issue to be addressed inthe automobile field, since there has been progress in reducing thethickness of automobile parts by using a high-strength steel sheet forautomobile parts, there is a trend toward using a steel sheet having atensile strength (TS) of 980 MPa or more. When a high-strength steelsheet which is used for the structural members and reinforcement membersof an automobile is subjected to forming in order to obtain parts havingcomplex shapes, the steel sheet is required to be excellent in terms offormability expressed not by a single property such as elongation (EL)or stretch flange formability (hereinafter, also referred to as “holeexpansion capability”) but by both of the properties. Moreover, in thecase where parts are assembled with high dimensional accuracy in orderto form a module by performing, for example, arc welding or spot weldingwithout allowing spring back or the like to occur after press forminghas been performed, it is important that a steel sheet have a low yieldratio (YR) before work is performed. Here, the term “yield ratio (YR)”refers to the ratio of yield stress (YS) to tensile strength (TS), andYR is expressed as YR=YS/TS.

Conventionally, examples of known high-strength cold-rolled steel sheethaving satisfactory formability and a high strength at the same timeinclude a dual-phase steel sheet (DP steel sheet), which has amulti-phase microstructure composed of ferrite and martensite. However,although a DP steel sheet has a high elongation (EL), a DP steel sheethas a disadvantage in that, since a crack tends to occur due to stressconcentration occurring at the interface between ferrite and martensite,there is a deterioration in bendability and hole expansion capability.Therefore, for example, Patent Literature 1 discloses a DP steel sheetin which the crystal grain diameter, volume fraction, andnanoindentation hardness of ferrite are controlled, and it is possibleto achieve a high elongation (EL) and excellent bendability with this DPsteel sheet.

In addition, examples of a steel sheet having a high strength and a highelongation (EL) at the same time include TRIP steel sheet. Since thisTRIP steel sheet has a steel sheet microstructure including retainedaustenite, a large elongation (EL) is achieved in the case where thesteel sheet is deformed by work performed at a temperature equal to orhigher than a temperature at which martensite transformation begins,because retained austenite transforms into martensite throughtransformation induced by stress. However, in the case of this TRIPsteel sheet, there is a disadvantage in that, since retained austenitetransforms into martensite when punching work is performed, a crackoccurs at the interface with ferrite, which results in a deteriorationin hole expansion capability. Therefore, for example, Patent Literature2 discloses a TRIP steel sheet which includes bainitic ferrite in orderto improve hole expansion capability.

PATENT LITERATURE

PTL 1: Japanese Patent No. 4925611

PTL 2: Japanese Patent No. 4716358

SUMMARY OF THE INVENTION

However, since the steel sheet disclosed in Patent Literature 1 has aninsufficient elongation (EL) in the case where it has a tensile strength(TS) of 980 MPa or more, it cannot be said that sufficient formabilityis achieved. In addition, since the steel sheet disclosed in PatentLiterature 2 which utilizes retained austenite has a yield ratio (YR) ofmore than 66% in the case where it has a tensile strength (TS) of 980MPa or more, spring back tends to occur after work has been performed.As described above, in the case of a high-strength steel sheet having atensile strength (TS) of 980 MPa or more, it is difficult to achieve ahigh elongation (EL) and a high hole expansion ratio (λ), which arerequired for increasing press formability (hereinafter, also referred toas “formability”), while maintaining a low yield ratio (YR), and it is afact that a steel sheet which is fully satisfactory in terms of theseproperties (yield ratio (YR), tensile strength (TS), elongation (EL),and hole expansion ratio (λ)) has not yet been developed.

Therefore, an object of certain embodiments of the present invention is,by solving the problems described above, to provide a high-strengthcold-rolled steel sheet excellent in terms of elongation (EL) and holeexpansion ratio (λ) having a low yield ratio (YR) and a method formanufacturing the steel sheet.

It is possible to achieve a high elongation (EL) and a high holeexpansion ratio (λ) while maintaining a low yield ratio (YR) bycontrolling the crystal grain diameters and volume fractions of steelsheet microstructures, that is, ferrite, retained austenite, andtempered martensite.

Generally, since movable dislocations are formed in ferrite whenmartensite transformation occurs in DP steel, DP steel has a low yieldratio (YR). However, since such martensite is hard, voids are formed atits interface, in particular, its interface with soft ferrite whenpunching work is performed in a hole expansion process, the voids thencombine with each other when the punched hole is expanded, and a crackoccurs as the combination of the voids progresses. Therefore, there is adecrease in the hole expansion ratio (λ) of DP steel. On the other hand,although there is an increase in hole expansion ratio (λ) by temperingmartensite, there is also an increase in yield ratio (YR) at the sametime. In addition, although retained austenite significantly increaseselongation (EL), since retained austenite, as is the case with hardmartensite, causes the formation of voids when punching work isperformed in a hole expansion process, there is a decrease in holeexpansion ratio (λ). As described above, it is conventionally difficultto improve the balance among elongation (EL), hole expansion ratio (λ),and yield ratio (YR).

Tempering conditions used for forming tempered martensite in order toincrease hole expansion ratio (λ) while achieving low yield ratio (YR)have been identified. Moreover, it is possible to inhibit thecombination of voids in a hole expansion process by decreasing theaverage crystal grain diameter of retained austenite and temperedmartensite in order to form a steel sheet microstructure in whichretained austenite and tempered martensite are finely dispersed, whichresults in an increase in elongation (EL) and hole expansion ratio (λ).In order to realize such an effect, fine martensite and retainedaustenite are formed by forming a microstructure composed of bainite andmartensite in a first annealing process after cold rolling has beenperformed, by forming fine austenite through reverse transformation in asecond annealing process, by then allowing bainite transformation tooccur through cooling, and by then performing rapid cooling. Moreover,by performing tempering on hard martensite in order to form temperedmartensite, it is possible to obtain a steel sheet having a highelongation (EL) and a high hole expansion ratio (λ) while achieving alow yield ratio (YR).

Certain embodiments of the present invention are as follows.

[1] A high-strength, cold-rolled steel sheet having

a chemical composition containing, by mass %, C: 0.15% to 0.25%, Si:1.0% to 2.0%, Mn: 1.8% to 2.5%, P: 0.10% or less, S: 0.010% or less, Al:0.10% or less, N: 0.010% or less, and the balance being Fe andinevitable impurities, and

a multi-phase microstructure including ferrite having an average crystalgrain diameter of 5 μm or less in an amount of 30% to 55% in terms ofvolume fraction, retained austenite having an average crystal graindiameter of 2 μm or less in an amount of 5% to 15% in terms of volumefraction, and tempered martensite having an average crystal graindiameter of 2 μm or less in an amount of 30% to 60% in terms of volumefraction,

in which the number of grains of the retained austenite existing in anarea of 1000 μm² is 10 or more.

[2] The high-strength, cold-rolled steel sheet according to item [1]above, in which the chemical composition further contains, by mass %,one or more selected from V: 0.10% or less, Nb: 0.10% or less, and Ti:0.10% or less.

[3] The high-strength, cold-rolled steel sheet according to item [1] or[2] above, in which the chemical composition further contains, by mass%, B: 0.010% or less.

[4] The high-strength, cold-rolled steel sheet according to any one ofitems [1] to [3] above, in which the chemical composition furthercontains, by mass %, one or more selected from Cr: 0.50% or less, Mo:0.50% or less, Cu: 0.50% or less, Ni: 0.50% or less, Ca: 0.0050% orless, and REM: 0.0050% or less.

[5] A method for manufacturing the high-strength, cold-rolled steelsheet according to any one of items [1] to [4] above,

the method including, after having performed hot rolling and coldrolling on a steel slab to obtain a cold-rolled steel sheet, performingcontinuous annealing on the cold-rolled steel sheet,

the continuous annealing comprising:

heating the cold-rolled steel sheet to a temperature of 850° C. orhigher,

holding the cold-rolled steel sheet at a first soaking temperature of850° C. or higher for 30 seconds or more,

then cooling the cold-rolled steel sheet from the first soakingtemperature to a temperature of 320° C. to 500° C. at a first averagecooling rate of 3° C./s or more,

holding the cold-rolled steel sheet at a second soaking temperature of320° C. to 500° C. for 30 seconds or more,

then cooling the cold-rolled steel sheet to a temperature of 100° C. orlower,

heating the cold-rolled steel sheet to a temperature of 750° C. orhigher at an average heating rate of 3° C./s to 30° C./s,

holding the cold-rolled steel sheet at a third soaking temperature of750° C. or higher for 30 seconds or more,

cooling the cold-rolled steel sheet from the third soaking temperatureto a temperature of 350° C. to 500° C. at a second average cooling rateof 3° C./s or more,

then cooling the cold-rolled steel sheet to a temperature of 100° C. orlower at a third average cooling rate of 100° C./s to 1000° C./s,

heating the cold-rolled steel sheet to a temperature of 200° C. to 350°C., and

then holding the cold-rolled steel sheet at a fourth soaking temperatureof 200° C. to 350° C. for 120 seconds to 1200 seconds.

In embodiments of the present invention, the term “a high-strengthcold-rolled steel sheet” refers to a cold-rolled steel sheet having atensile strength (TS) of 980 MPa or more.

In addition, in embodiments of the present invention, the term “anaverage cooling rate” refers to a value derived by dividing a valuederived by subtracting a cooling stop temperature from a cooling starttemperature by a cooling time. In addition, the term “an average heatingrate” refers to a value derived by dividing a value derived bysubtracting a heating start temperature from a heating stop temperatureby a heating time.

According to embodiments of the present invention, by controlling thechemical composition and microstructure of a steel sheet, it is possibleto stably obtain a high-strength cold-rolled steel sheet having a highelongation (EL) and a high hole expansion ratio (λ), that is, ahigh-strength cold-rolled steel sheet having a tensile strength (TS) of980 MPa or more, a low yield ratio (YR) of 66% or less, an elongation(EL) of 19% or more, and a hole expansion ratio (λ) of 30% or more.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Hereafter, embodiments of the present invention will be specificallydescribed. The high-strength, cold-rolled steel sheet according to thepresent invention has a chemical composition containing, by massa, C:0.15% to 0.25%, Si: 1.0% to 2.0%, Mn: 1.8% to 2.5%, P: 0.10% or less, S:0.010% or less, Al: 0.10% or less, N: 0.010% or less, and the balancebeing Fe and inevitable impurities, and a multi-phase microstructureincluding ferrite having an average crystal grain diameter of 5 μm orless in an amount of 30% to 55% in terms of volume fraction, retainedaustenite having an average crystal grain diameter of 2 μm or less in anamount of 5% to 15% in terms of volume fraction, and tempered martensitehaving an average crystal grain diameter of 2 μm or less in an amount of30% to 60% in terms of volume fraction, in which the number of grains ofthe retained austenite existing in an area of 1000 μm² is 10 or more.

First, the reasons for the limitations on the chemical element of thehigh-strength cold-rolled steel sheet according to embodiments of thepresent invention will be described. Hereinafter, “%” used whendescribing a chemical composition shall refer to “mass %”.

C: 0.15% to 0.25%

C is a chemical element which is effective for increasing the strengthof a steel sheet and which also contributes to the formation of secondphases in embodiments of the present invention, that is, temperedmartensite and retained austenite. In the case where the C content isless than 0.15%, since it is difficult to control the volume fraction oftempered martensite to be 30% or more or the volume fraction of retainedaustenite to be 5% or more, it is difficult to control tensile strength(TS) to be 980 MPa or more. Therefore, the C content is set to be 0.15%or more, or preferably 0.18% or more. On the other hand, in the casewhere the C content is more than 0.25%, since there is an increase inthe difference in hardness between ferrite and tempered martensite, itis not possible to achieve the desired hole expansion ratio W.Therefore, the C content is set to be 0.25% or less, or preferably 0.23%or less.

Here, in the case of the high-strength cold-rolled steel sheet accordingto embodiments of the present invention, the term “a main phase” refersto a ferrite phase, and the term “second phases” described above refersto a tempered martensite phase and a retained austenite phase. Inaddition, the microstructure of the high-strength cold-rolled steelsheet according to embodiments of the present invention may includetempered bainite and pearlite.

Si: 1.0% to 2.0%

Si is a chemical element which is necessary for contributing to theformation of retained austenite by inhibiting the formation of carbideswhen bainite transformation occurs in the first and second annealingprocesses. In the case where the Si content is less than 1.0%, it is notpossible to form a sufficient amount of retained austenite. Therefore,the Si content is set to be 1.0% or more, or preferably 1.3% or more. Onthe other hand, in the case where the Si content is more than 2.0%,since the volume fraction of ferrite becomes more than 55%, and sincethe average crystal grain diameter becomes more than 5 μm, it is notpossible to achieve a tensile strength (TS) of 980 MPa or more or asufficient hole expansion ratio (λ). Therefore, the Si content is set tobe 2.0% or less, or preferably 1.8% or less.

Mn: 1.8% to 2.5%

Mn is a chemical element which contributes to an increase in strengththrough solid solution strengthening and by facilitating the formationof the second phases and which stabilizes austenite. In the case wherethe Mn content is less than 1.8%, it is not possible to control thevolume fraction of the second phases to be within the desired ranges.Therefore, the Mn content is set to be 1.8% or more. On the other hand,in the case where the Mn content is more than 2.5%, since the volumefraction of tempered martensite becomes more than 60%, and since thereis an increase in the hardness of tempered martensite, it is notpossible to achieve the desired hole expansion ratio (λ). Therefore, theMn content is set to be 2.5% or less.

P: 0.10% or less

Although P contributes to an increase in strength through solid solutionstrengthening, in the case where the P content is more than 0.10%, sincethe segregation of P significantly occurs at the grain boundaries, agrain-boundary crack occurs due to the embrittlement of the grainboundaries, which makes it impossible to achieve the desired holeexpansion ratio (λ). Therefore, the P content is set to be 0.10% orless, or preferably 0.05% or less.

S: 0.010% or less

In the case where the S content is more than 0.010%, since large amountsof sulfides such as MnS are formed, voids are formed around the sulfideswhen punching is performed in a hole expansion test, which makes itimpossible to achieve the desired hole expansion ratio (λ). Therefore,the S content is set to be 0.010% or less, or preferably 0.005% or less.On the other hand, although there is no particular limitation on thelower limit of the S content, there is an increase in steel-making costsin order to control the S content to be very small, that is, less than0.0005%. Therefore, it is preferable that the S content be 0.0005% ormore.

Al: 0.10% or less

Although Al is a chemical element which is necessary for deoxidation, inthe case where the Al content is more than 0.10%, this deoxidationeffect becomes saturated. Therefore, the Al content is set to be 0.10%or less, or preferably 0.08% or less. On the other hand, in order torealize this deoxidation effect, it is preferable that the Al content be0.01% or more.

N: 0.010% or less

Since N decreases the hole expansion ratio (λ) by forming coarsenitrides, it is necessary to decrease the N content. In the case wherethe N content is more than 0.010%, it is not possible to achieve thedesired hole expansion ratio (λ). Therefore, the N content is set to be0.010% or less, or preferably 0.006% or less.

The remainder which is different from the constituent chemical elementsdescribed above is Fe and inevitable impurities. Examples of theinevitable impurities include Sb, Sn, Zn, and Co, and the acceptableranges of the contents of these chemical elements are respectively Sb:0.01% or less, Sn: 0.10% or less, Zn: 0.01% or less, and Co: 0. 10% orless.

In addition, even in the case where Ta, Mg, and Zr are added in amountswithin the ranges which are common among ordinary steel chemicalcompositions, there is no decrease in the effects of certain embodimentsof the present invention.

In addition, in embodiments of the present invention, one, two, or moreof the following chemical elements may be added in addition to theconstituent chemical elements described above.

V: 0.10% or less

Since V contributes to an increase in strength by forming finecarbonitrides, V may be added as needed. In order to realize such aneffect, it is preferable that the V content be 0.01% or more. On theother hand, in the case where the V content is large, there is only asmall increase in the effect of increasing strength corresponding to anincrease in V content in the case where the V content is more than0.10%, and there is an increase in alloy costs. Therefore, in the casewhere V is added, it is preferable that the V content be 0.10% or less.

Nb: 0.10% or less

Since Nb, like V, contributes to an increase in strength by forming finecarbonitrides, Nb may be added as needed. In order to realize such aneffect, it is preferable that the Nb content be 0.005% or more. On theother hand, there is a significant deterioration in elongation (EL) inthe case where the Nb content is more than 0.10%. Therefore, it ispreferable that the Nb content be 0.10% or less.

Ti: 0.10% or less

Since Ti, like V, contributes to an increase in strength by forming finecarbonitrides, Ti may be added as needed. In order to realize such aneffect, it is preferable that the Ti content be 0.005% or more. On theother hand, in the case where the Ti content is more than 0.10%, thereis a significant deterioration in elongation (EL). Therefore, it ispreferable that the Ti content be 0.10% or less.

B: 0.010% or less

Since B is a chemical element which contributes to an increase instrength by increasing hardenability and by facilitating the formationof the second phases and which achieves hardenability withoutsignificantly increasing the hardness of tempered martensite, B may beadded as needed. In order to realize such effects, it is preferable thatthe B content be 0.0003% or more. On the other hand, in the case wherethe B content is more than 0.010%, such effects become saturated.Therefore, it is preferable that the B content be 0.010% or less.

Cr: 0.50% or less

Since Cr is a chemical element which contributes to an increase instrength by facilitating the formation of the second phases, Cr may beadded as needed. In order to realize such an effect, it is preferablethat the Cr content be 0.10% or more. On the other hand, in the casewhere the Cr content is more than 0.50%, an excessive amount of temperedmartensite is formed. Therefore, in the case where Cr is added, it ispreferable that the Cr content be 0.50% or less.

Mo: 0.50% or less

Since Mo is a chemical element which contributes to an increase instrength by facilitating the formation of the second phases and bypartially forming carbides, Mo may be added as needed. In order torealize such an effect, it is preferable that the Mo content be 0.05% ormore. On the other hand, in the case where the Mo content is more than0.50%, such an effect becomes saturated. Therefore, in the case where Mois added, it is preferable that the Mo content be 0.50% or less.

Cu: 0.50% or less

Since Cu is a chemical element which contributes to an increase instrength through solid solution strengthening and by facilitating theformation of the second phases, Cu may be added as needed. In order torealize such an effect, it is preferable that the Cu content be 0.05% ormore. On the other hand, in the case where the Cu content is more than0.50%, such an effect becomes saturated, and surface defects caused byCu tend to occur. Therefore, in the case where Cu is added, it ispreferable that the Cu content be 0.50% or less.

Ni: 0.50% or less

Since Ni is, like Cu, a chemical element which contributes to anincrease in strength through solid solution strengthening and byfacilitating the formation of the second phases, Ni may be added asneeded. In order to realize such an effect, it is preferable that the Nicontent be 0.05% or more. In addition, in the case where Ni is added incombination with Cu, since Ni is effective for inhibiting surfacedefects caused by Cu from occurring, adding Ni is effective when Cu isadded. On the other hand, in the case where the Ni content is more than0.50%, such effects become saturated. Therefore, in the case where Ni isadded, it is preferable that the Ni content be 0.50% or less.

Ca: 0.0050% or less

Since Ca contributes to inhibiting a decrease in the hole expansionratio (λ) due to sulfides by spheroidizing the shape of sulfides, Ca maybe added as needed. In order to realize such an effect, it is preferablethat the Ca content be 0.0005% or more. On the other hand, in the casewhere the Ca content is more than 0.0050%, such an effect becomessaturated. Therefore, in the case where Ca is added, it is preferablethat the Ca content be 0.0050% or less.

REM: 0.0050% or less

Since REM, like Ca, contributes to inhibiting a decrease in the holeexpansion ratio (λ) due to sulfides by spheroidizing the shape ofsulfides, REM may be added as needed. In order to realize such aneffect, it is preferable that the REM content be 0.0005% or more. On theother hand, in the case where the REM content is more than 0.0050%, suchan effect becomes saturated. Therefore, in the case where REM is added,it is preferable that the REM content be 0.0050% or less.

Hereafter, the microstructure of the high-strength cold-rolled steelsheet according to embodiments of the present invention will bedescribed in detail. The high-strength cold-rolled steel sheet accordingto embodiments of the present invention includes ferrite, retainedaustenite, and tempered martensite. In addition, the high-strengthcold-rolled steel sheet according to embodiments of the presentinvention may include tempered bainite as the remainder of themicrostructure. The ferrite has an average crystal grain diameter of 5μm or less and a volume fraction of 30% to 55%. In addition, theretained austenite has an average crystal grain diameter of 2 μm or lessand a volume fraction of 5% to 15%. In addition, the tempered martensitehas an average crystal grain diameter of 2 μm or less and a volumefraction of 30% to 60%. In addition, in the case of the high-strengthcold-rolled steel sheet according to embodiments of the presentinvention, the number of grains of retained austenite having an averagecrystal grain diameter of 2 μm or less existing in an area of 1000 μm²is 10 or more. The term “a volume fraction” here shall refer to a volumefraction with respect to the whole steel sheet, and the same shall applyhereinafter.

In the case where the volume fraction of ferrite described above is lessthan 30%, since there is an insufficient amount of soft ferrite, thereis a decrease in elongation (EL). Therefore, the volume fraction offerrite is set to be 30% or more, or preferably 35% or more. On theother hand, in the case where the volume fraction of ferrite is morethan 55%, it is difficult to achieve a tensile strength (TS) of 980 MPaor more. Therefore, the volume fraction of ferrite is set to be 55% orless, or preferably 50% or less. In addition, in the case where theaverage crystal grain diameter of ferrite is more than 5 μm, since voidswhich have been formed in a punched end surface in a hole expansionprocess tend to combine with each other when the punched hole isexpanded, it is not possible to achieve the desired hole expansion ratio(?). Moreover, in the case where the average crystal grain diameter offerrite is more than 5 μm, it is not possible to achieve a yield ratio(YR) of less than the desired value. Therefore, the average crystalgrain diameter of ferrite is set to be 5 μm or less.

In order to achieve a high elongation (EL), it is necessary that thevolume fraction of retained austenite be 5% to 15%. In the case wherethe volume fraction of retained austenite is less than 5%, it is notpossible to achieve the desired elongation (EL). Therefore, the volumefraction of retained austenite is set to be 5% or more, or preferably 6%or more. On the other hand, in the case where the volume fraction ofretained austenite is more than 15%, it is not possible to achieve thedesired hole expansion ratio (λ). Therefore, the volume fraction ofretained austenite is set to be 15% or less, or preferably 12% or less.In addition, in order to achieve a high hole expansion ratio (λ), theaverage crystal grain diameter of retained austenite is set to be 2 μmor less. In the case where the average crystal grain diameter ofretained austenite is more than 2 μm, voids tend to combine with eachother after the voids have been formed in a hole expansion process.Therefore, the average crystal grain diameter of retained austenite isset to be 2 μm or less.

In order to achieve a tensile strength of 980 MPa or more whileachieving the desired hole expansion ratio (λ) and low yield ratio (YR),the volume fraction of tempered martensite is set to be 30% to 60%. Inthe case where the volume fraction of tempered martensite is less than30%, it is not possible to achieve a tensile strength of 980 MPa ormore. On the other hand, in the case where the volume fraction oftempered martensite is more than 60%, it is difficult to achieve thedesired elongation (EL). In addition, in order to achieve a high holeexpansion ratio (λ), the average crystal grain diameter of temperedmartensite is set to be 2 μm or less. In the case where the averagecrystal grain diameter is more than 2 μm, since voids which have beenformed at the grain boundaries with ferrite tend to combine with eachother, it is not possible to achieve the desired hole expansion ratio(λ). Therefore, the upper limit of the average crystal grain diameter oftempered martensite is ,set to be 2 μm.

In addition, in a steel sheet microstructure, tempered bainite may bepartially formed in order to form retained austenite by allowing bainitetransformation to occur in an annealing process. Although there is noparticular limitation on the volume fraction of this tempered bainite,it is preferable that the volume fraction of tempered bainite be 30% orless in order to achieve a high elongation (EL).

Moreover, in order to achieve a high elongation (EL), it is necessarythat the number of grains of the above-described retained austenitehaving an average crystal grain diameter of 2 μm or less existing in anarea of 1000 μm² be 10 or more. In the case where the number of grainsof retained austenite existing in an area of 1000 μm² is less than 10,it is not possible to achieve the desired elongation (EL). On the otherhand, although there is no particular limitation on the upper limit ofthe number of grains of retained austenite existing in an area of 1000μm², in the case where the number of grains of retained austeniteexisting in an area of 1000 μm² is more than 50, voids which have beenformed at the grain boundaries with ferrite tend to combine with eachother. Therefore, it is preferable that the number of grains of retainedaustenite existing in an area of 1000 μm² be 50 or less.

In addition, in the case of the steel sheet according to embodiments ofthe present invention, although there is a case where tempered bainiteand pearlite are formed in addition to ferrite, retained austenite, andtempered martensite, it is possible to achieve an object of the presentinvention as long as the above-described conditions regarding the volumefractions and average crystal grain diameters of ferrite, retainedaustenite, and tempered martensite and the number of grains of retainedaustenite existing in an area of 1000 μm² are satisfied. However, it ispreferable that the volume fraction of pearlite be 5% or less. Inaddition, as described above, it is preferable that the volume fractionof tempered bainite be 30% or less.

Here, it is possible to observe the multi-phase microstructure of asteel sheet described above by using, for example, a SEM (scanningelectron microscope). Specifically, first, a cross section in thethickness direction parallel to the rolling direction of a steel sheetis polished and then etched by using nital (an alcohol solutioncontaining nitric acid). Subsequently, by taking microstructurephotographs through the use of a scanning electron microscope atmagnifications of 2000 times and 5000 times, by extracting desiredregions in the obtained microstructure photograph data through imageanalysis, it is possible to identify ferrite, retained austenite,tempered martensite, or tempered bainite through the use of imageanalysis software (Image-Pro ver. 7 produced by Media Cybernetics,Inc.).

It is possible to determine the above-described desired volume fractionsof ferrite, retained austenite, and tempered martensite by determiningthe area ratios of these phases through the use of a point-countingmethod (in accordance with ASTM E562-83 (1988)) and by defining the arearatios as the volume fractions. In addition, it is possible to determinethe above-described desired average crystal grain diameters of ferrite,retained austenite, and tempered martensite by calculating thecircle-equivalent diameters of these phases from a steel sheetmicrostructure photograph and by calculating the average values of thecircle-equivalent diameters. In addition, it is possible to determinethe number of grains of retained austenite by counting in theobservation of a steel sheet microstructure photograph.

In addition, it is possible to control the above-described desiredvolume fractions and average crystal grain diameters of ferrite,retained austenite, and tempered martensite and the number of grains ofretained austenite by controlling a steel sheet microstructure when thefirst annealing is performed and/or when the second annealing isperformed.

Hereafter, the method for manufacturing the high-strength cold-rolledsteel sheet according to embodiments of the present invention will bedescribed.

The method for manufacturing the high-strength cold-rolled steel sheetaccording to embodiments of the present invention includes, after havingperformed hot rolling and cold rolling on a steel slab having thechemical composition (constituent chemical elements) described above,performing continuous annealing on the cold-rolled steel sheet, in whichheating is performed to a temperature of 850° C. or higher, in whichholding is performed at a first soaking temperature of 850° C. or higherfor 30 seconds or more, in which cooling is then performed from thefirst soaking temperature to a second soaking temperature of 320° C. to500° C. at a first average cooling rate of 3° C./s or more, in whichholding is performed at the second soaking temperature of 320° C. to500° C. for 30 seconds or more, in which cooling is then performed to atemperature of 100° C. or lower (for example, room temperature), inwhich heating is thereafter performed to a temperature of 750° C. orhigher at an average heating rate of 3° C./s to 30° C./s, in whichholding is performed at a third soaking temperature of 750° C. or higherfor 30 seconds or more, in which cooling is then performed from thethird soaking temperature to a temperature of 350° C. to 500° C. at asecond average cooling rate of 3° C./s or more, in which cooling isperformed to a temperature of 100° C. or lower at a third averagecooling rate of 100° C./s to 1000° C./s, in which heating is performedto a temperature of 200° C. to 350° C., and in which holding is thenperformed at a fourth soaking temperature of 200° C. to 350° C. for 120seconds to 1200 seconds.

[Hot Rolling Process]

In the hot rolling process, by performing rough rolling and finishrolling on a steel slab having the chemical composition described aboveafter heating has been performed, a hot-rolled steel sheet is obtained.Although it is preferable that the steel slab used be manufactured byusing a continuous casting method in order to prevent the macrosegregation of the constituent chemical elements, the slab may also bemanufactured by using an ingot-making method or a thin-slab-castingmethod. Regarding preferable hot rolling conditions, first, the castslab may not be reheated or may be reheated to a temperature of 1100° C.or higher. In embodiments of the present invention, in addition to aconventional method, in which, after having manufactured a steel slab,the slab is first cooled to a temperature of 100° C. or lower (forexample room temperature) and then reheated, an energy-saving processsuch as a hot direct rolling or a direct rolling, that is, a method inwhich a slab in the hot state is charged into a heating furnace withoutthe slab having been cooled, a method in which a slab is rolledimmediately after heat retention has been performed, or a method inwhich a slab in the cast state is rolled may be used without causing anyproblem.

By controlling a slab heating temperature to be 1100° C. or higher, itis possible to decrease rolling load and to improve productivity. On theother hand, by controlling the slab heating temperature to be 1300° C.or lower, it is possible to decrease heating costs. Therefore, it ispreferable that the slab heating temperature be 1100° C. to 1300° C.

In addition, by controlling a finishing delivery temperature to be 830°C. or higher, since it is possible to finish hot rolling within anaustenite single phase region, it is possible to inhibit a decrease inelongation (EL) and hole expansion ratio (λ) due to an increase in theinhomogeneity of a microstructure in a steel sheet and the anisotropy ofmaterial properties after annealing. On the other hand, by controllingthe finishing delivery temperature to be 950° C. or lower, it ispossible to inhibit deterioration in properties after annealing due tocoarsening of a hot-rolled microstructure. Therefore, it is preferablethat the finishing delivery temperature be 830° C. to 950° C.

There is no particular limitation on the method used for cooling thehot-rolled steel sheet after hot rolling. Also, there is no particularlimitation on a coiling temperature.

However, by controlling a coiling temperature to be 700° C. or lower,since it is possible to inhibit the formation of coarse pearlite, it ispossible to prevent a decrease in elongation (EL) and hole expansionratio (λ) after annealing has been performed. Therefore, it ispreferable that the coiling temperature be 700° C. or lower, or morepreferably 650° C. or lower. On the other hand, although there is noparticular limitation on the lower limit of the coiling temperature, bycontrolling the coiling temperature to be 400° C. or higher, since it ispossible to inhibit the formation of excessive amounts of hard bainiteand martensite, it is possible to decrease cold rolling load. Therefore,it is preferable that the coiling temperature be 400° C. or higher.

[Pickling Process]

In the method for manufacturing the high-strength cold-rolled steelsheet according to embodiments of the present invention, pickling may beperformed on the hot-rolled steel sheet after the hot rolling processdescribed above. It is preferable that scale on the surface of thehot-rolled steel sheet be removed by performing pickling. There is noparticular limitation on the method used for pickling, and pickling maybe performed by using a commonly used method.

[Cold Rolling Process]

In the method for manufacturing the high-strength cold-rolled steelsheet according to embodiments of the present invention, after hotrolling has been performed on the steel slab described above, or afterpickling has been performed on the hot-rolled steel sheet, cold rolling,in which rolling is performed in order to obtain a cold-rolled steelsheet having a specified thickness, is performed. There is no particularlimitation on the cold rolling process, and cold rolling may beperformed by using a commonly used method. In addition, intermediateannealing may be performed before the cold rolling process. Byperforming intermediate annealing, it is possible to decrease coldrolling load. Although there is no particular limitation on the time orthe temperature of the intermediate annealing, in the case where batchannealing is performed on a steel sheet in a coiled state, for example,it is preferable that annealing be performed at a temperature of 450° C.to 800° C. for 10 minutes to 50 hours.

[Annealing Process]

In the method for manufacturing the high-strength cold-rolled steelsheet according to embodiments of the present invention, after coldrolling as described above, annealing is performed on the cold-rolledsteel sheet. In the annealing process, recrystallization is progressed,and retained austenite and tempered martensite are formed in a steelsheet microstructure in order to increase strength. In addition, in themethod for manufacturing the high-strength cold-rolled steel sheetaccording to embodiments of the present invention, by performingannealing twice, since it is possible to make fine crystal grains oftempered martensite and retained austenite after annealing, it ispossible to achieve a high hole expansion ratio (λ). By allowinguntransformed austenite to transform into bainite during a coolingprocess in the first annealing process, large amounts of fine retainedaustenite and martensite are retained. However, since the crystal graindiameter of martensite is still large after only the first annealingprocess has been performed, it is not possible to achieve the desiredhole expansion ratio (λ). Therefore, the second annealing is performedin order to further make fine crystal grains of martensite. With this,since martensite and retained austenite, which have been formed in thefirst annealing process, become the nucleation sites of austenite whichis formed through reverse transformation in the second annealingprocess, it is possible to perform cooling while maintaining fine phasesin the second annealing process. That is, by forming a steel sheetmicrostructure including bainite, martensite, and retained austenitewhich are homogenized to some extent in the first annealing process, itis possible to allow more homogeneous fine dispersion to occur in thesecond annealing process. In order to form tempered martensite in thesecond annealing process, tempering is performed after cooling is firstperformed to an excessive degree. With this, it is possible to achieve ahigh hole expansion ratio (λ) while inhibiting a decrease in elongation(EL).

Therefore, in the first annealing process, heating is performed to atemperature of 850° C. or higher, holding is performed at a firstsoaking temperature of 850° C. or higher for 30 seconds or more, coolingis then performed from the first soaking temperature to a second soakingtemperature of 320° C. to 500° C. at a first average cooling rate of 3°C./s or more, holding is performed at the second soaking temperature of320° C. to 500° C. for 30 seconds or more, and cooling is then performedto a temperature of 100° C. or lower (for example, room temperature).Thereafter, in the second annealing process, heating is performed to atemperature of 750° C. or higher at an average heating rate of 3° C./sto 30° C./s, holding is performed at a third soaking temperature of 750°C. or higher for 30 seconds or more, cooling is then performed from thethird soaking temperature to a temperature of 350° C. to 500° C. at asecond average cooling rate of 3° C./s or more, cooling is performed toa temperature of 100° C. or lower at a third average cooling rate of100° C./s to 1000° C./s, heating is performed to a temperature of 200°C. to 350° C., and holding is then performed at a fourth soakingtemperature of 200° C. to 350° C. for 120 seconds to 1200 seconds.

<First Annealing Process>

(Heating to First Soaking Temperature (850° C. or Higher) and Holdingfor 30 Seconds or More)

In the first annealing process, heating is firstly performed to thefirst soaking temperature. This first soaking temperature is set to be atemperature in a temperature range in which an austenite single phase isformed. In the case where the first soaking temperature is lower than850° C., since there is a decrease in the amount of bainite after thefirst annealing process, there is an increase in the crystal graindiameter of tempered martensite and retained austenite which are formedin the second annealing process, which results in a decrease in holeexpansion ratio (λ). Therefore, the lower limit of the first soakingtemperature is set to be 850° C., or preferably 870° C. or higher. Inaddition, it is preferable that the first soaking temperature be 1000°C. or lower in order to prevent the crystal grain diameter of austenitefrom increasing. In addition, in order to allow recrystallization toprogress and in order to allow the all or part of the grains totransform into austenite, the holding time (soaking time) at the firstsoaking temperature is set to be 30 seconds or more. Although there isno particular limitation on the upper limit of this holding time, it ispreferable that this holding time be 600 seconds or less in order toprevent coarse carbides from being formed in a steel sheet.

(Cooling from First Soaking Temperature to Second Soaking Temperature(320° C. to 500° C.) at First Average Cooling Rate (3° C./s or More))

In the first annealing process, in order to form a steel sheetmicrostructure including a large amount of bainite, cooling is performedto a second soaking temperature of 320° C. to 500° C. at a first averagecooling rate of 3° C./s or more. In the case where the first averagecooling rate is less than 3° C./s, since excessive amounts of ferrite,pearlite and spherical cementite are formed in a steel sheetmicrostructure, the lower limit of the first average cooling rate is setto be 3° C./s. In addition, in the case where the cooling stoptemperature (hereinafter, also referred to as “second soakingtemperature”) is lower than 320° C., since an excessive amount ofmassive martensite is formed in the cooling process, it is difficult toform a fine homogeneous steel sheet microstructure in the secondannealing process, which makes it impossible to achieve the desired holeexpansion ratio (λ). In the case where the cooling stop temperature(second soaking temperature) is higher than 500° C., since there is anexcessive increase in the amount of pearlite, it is difficult to form afine homogeneous steel sheet microstructure in the second annealingprocess, which makes it impossible to achieve the desired hole expansionratio (λ). Therefore, the second soaking temperature is set to be 320°C. to 500° C., or preferably 350° C. to 450° C.

(Holding at Second Soaking Temperature for 30 Seconds or More)

By allowing untransformed austenite to transform into bainite, bainiteand retained austenite are formed. In the case where holding time at thesecond soaking temperature is less than 30 seconds, since there is anincrease in the amount of untransformed austenite, there is an increasein the amount of massive martensite in a steel sheet microstructureafter the first annealing process, which makes it impossible to makefine crystal grains of a steel sheet microstructure after the secondannealing process. Therefore, the holding time at the second soakingtemperature is set to be 30 seconds or more.

(Cooling to a Temperature of 100° C. or Lower)

After holding has been performed at the second soaking temperature,cooling is performed to a temperature of 100° C. or lower (for example,room temperature). With this, it is possible to form a steel sheetmicrostructure including bainite.

<Second Annealing Process>

(Heating to Third Soaking Temperature (750° C. or higher) at an AverageHeating Rate of 3° C./s to 30° C./s)

In the second annealing process, by forming the nucleation sites offerrite and austenite which are formed by recrystallization due toreverse transformation, and by controlling the speed of the formation ofthe nucleation sites of recrystallized ferrite to be larger than that ofcoarsening of the nucleation sites in order to grow grains, it ispossible to make the crystal grains fine after the annealing process. Inthe case where the third soaking temperature is lower than 750° C.,since there is an excessively small amount of austenite formed, it isnot possible to achieve the desired volume fractions of martensite andretained austenite formed. Therefore, the third soaking temperature isset to be 750° C. or higher. In addition, it is preferable that thethird soaking temperature be 900° C. or lower in order to remove theinfluence of the steel sheet microstructure which has been formed in thefirst annealing process by performing annealing in an austenite singlephase region is formed. In addition, in the case where the averageheating rate to the third soaking temperature (750° C. or higher) ismore than 30° C./s, recrystallization is less likely to progress.Therefore, the average heating rate is set to be 30° C./s or less. Inaddition, in the case where the average heating rate to the thirdsoaking temperature (750° C. or higher) is less than 3° C./s, sinceferrite grains are coarsened, it is not possible to achieve thespecified average crystal grain diameter. Therefore, the average heatingrate is set to be 3° C./s or more.

(Holding at Third Soaking Temperature for 30 Seconds or More)

In the second annealing process, holding is performed at a third soakingtemperature of 750° C. or higher for 30 seconds or more. In the casewhere the holding time at the third soaking temperature is less than 30seconds, since insufficient amounts of chemical elements such as Mn areconcentrated in austenite, and crystal grains of untransformed austeniteare coarsened in the cooling process, it is impossible to achieve thedesired hole expansion ratio (λ). Therefore, the holding time at thethird soaking temperature is set to be 30 seconds or more.

(Cooling from Third Soaking Temperature to a Temperature of 350° C. to500° C. at Second Average Cooling Rate (3° C./s or More))

In order to increase elongation (EL) by forming retained austenite,since it is necessary that the concentration of C and Mn inuntransformed austenite and bainite transformation be promoted in acooling process from the third soaking temperature, cooling is performedto a temperature of 350° C. to 500° C. at a second average cooling rateof 3° C./s or more. In the case where the second average cooling rate isless than 3° C./s, excessive amounts of pearlite and spherical cementiteare formed in a steel sheet microstructure. Therefore, the lower limitof the second average cooling rate is set to be 3° C./s. In addition, inthe case where cooling at the second average cooling rate is performedto a temperature of lower than 350° C., since an excessive amount ofmartensite is formed in the cooling process, and the amounts of bainitetransformation and retained austenite are decreased due to a decrease inthe amount of untransformed austenite, it is impossible to achieve thedesired elongation (EL). Therefore, cooling at the second averagecooling rate should be performed to a temperature of 350° C. or higher.On the other hand, in the case where cooling at the second averagecooling rate is performed to a temperature of higher than 500° C., sinceinsufficient amounts of C and Mn are concentrated in untransformedaustenite, there is a decrease in the amount of retained austeniteformed in the final steel sheet microstructure, which makes itimpossible to achieve the desired elongation (EL). Therefore, thecooling at the second average cooling rate should be performed to atemperature of 500° C. or lower, or preferably 370° C. to 450° C.

(Cooling to a Tmperature of 100° C. or Lower at a Third Average CoolingRate of 100° C./s to 1000° C./s)

Subsequently, in order to form fine martensite and retained austenite,cooling is performed to a temperature of 100° C. or lower at a thirdaverage cooling rate of 100° C./s to 1000° C./s. In the case where thethird average cooling rate is less than 100° C./s, since an excessiveamount of bainite is formed, it is not possible to achieve the desiredvolume fractions. Therefore, the third average cooling rate is set to be100° C./s or more. On the other hand, in the case where the thirdaverage cooling rate is more than 1000° C./s, a shrinkage cracking mayoccur in a steel sheet due to cooling. Therefore, the third averagecooling rate is set to be 1000° C./s or less. Here, in the case of thiscooling, it is preferable that water quenching be performed.

(Tempering)

After cooling as described above, a tempering treatment is performed.This tempering is performed in order to improve workability by softeningmartensite. That is, after cooling is performed as described above, inorder to temper martensite, after heating has been performed to atemperature of 200° C. to 350° C., holding is performed at a temperingtemperature of 200° C. to 350° C. (hereinafter, also referred to as“fourth soaking temperature”) for 120 seconds to 1200 seconds. In thecase where the tempering temperature (fourth soaking temperature) islower than 200° C., since the softening of martensite is insufficient,there is a decrease in hole expansion capability. Therefore, the fourthsoaking temperature is set to be 200° C. or higher. On the other hand,the tempering temperature (fourth soaking temperature) is higher than350° C., there is an increase in yield ratio (YR). Therefore, the fourthsoaking temperature is set to be 350° C. or lower, or preferably 300° C.or lower. In addition, in the case where the holding time at the fourthsoaking temperature is less than 120 seconds, since there is aninsufficient improvement in the property of martensite at the fourthsoaking temperature, it is not possible to expect the effect ofincreasing hole expansion ratio (λ). Therefore, the holding time at thefourth soaking temperature is set to be 120 seconds or more. On theother hand, in the case where the holding time at the fourth soakingtemperature is more than 1200 seconds, there is a significant decreasein tensile strength due to the softening of martensite progressingexcessively, and there is an increase in manufacturing costs due to anincrease in reheating time. Therefore, the holding time at the fourthsoaking temperature is set to be 1200 seconds or less. Here, after theholding at the above-described temperature, there is no limitation oncooling method or cooling rate.

In addition, skin pass rolling may be performed after the annealingprocess. It is preferable that skin pass rolling be performed with anelongation ratio of 0.1% to 2.0%.

Here, within the range according to embodiments of the presentinvention, a galvanized steel sheet may be manufactured by performing agalvanizing treatment in the annealing process, and a galvannealed steelsheet may be manufactured by performing an alloying treatment after agalvanizing treatment has been performed. Moreover, by performing anelectroplating treatment on the cold-rolled steel sheet according toembodiments of the present invention, an electroplated steel sheet maybe manufactured.

EXAMPLES

Hereafter, the examples of embodiments of the present invention will bedescribed. However, the present invention is not originally limited bythe examples described below, and the present invention may be performedby appropriately making alterations within a range in accordance withthe intent of the present invention. Working examples performed in sucha way are all within the technical scope of the present invention.

By preparing molten steels having the chemical compositions given inTable 1, by casting the steels in order to manufacture slabs having athickness of 230 mm, by performing hot rolling under the conditions of ahot rolling heating temperature of 1200° C. and a finishing deliverytemperature of 900° C., by performing cooling to a temperature of 650°C. at a cooling rate of 100° C./s after hot rolling had been performed,and by then performing cooling at a cooling rate of 20° C./s, hot-rolledsteel sheets having a thickness of 3.2 mm were obtained and coiled at acoiling temperature of 600° C. Subsequently, by pickling the obtainedhot-rolled steel sheets, and by then performing cold rolling,cold-rolled steel sheets (having a thickness of 1.4 mm) weremanufactured. Subsequently, in the first annealing process, heating wasperformed to the first soaking temperatures given in Table 2, annealingwas performed at the first soaking temperatures for the first soakingtime (first holding time), cooling was the performed to the secondsoaking temperatures at the first average cooling rates (CR1) given inTable 2, holding was performed for the second soaking time (secondholding time), and then cooling was performed to room temperature (25°C.) Subsequently, in the second annealing process, heating was performedat the average heating rates given in Table 2, holding was performed atthe third soaking temperatures for the third soaking time (third holdingtime), cooling was then performed to the quenching start temperatures(cooling start temperatures of cooling performed at the third averagecooling rates, that is, cooling stop temperatures: Tq) at the secondaverage cooling rates (CR2) given in Table 2, cooling was then performedto room temperature (25° C.) at the third average cooling rates (CR3),and then, in the tempering process, heating was performed to the fourthsoaking temperatures given in Table 2, holding was performed for thefourth soaking times (fourth holding times given in Table 2), and thencooling was performed to room temperature (25° C.)

TABLE 1 Steel Chemical Composition (mass %) Grade C Si Mn P S Al N OtherNote A 0.20 1.45 2.21 0.01 0.002 0.03 0.002 — Example Steel B 0.18 1.562.19 0.01 0.001 0.03 0.003 Ti: 0.02 Example Steel C 0.21 1.81 2.31 0.010.001 0.03 0.003 V: 0.02 Example Steel D 0.22 1.44 1.98 0.01 0.001 0.030.002 Nb: 0.02 Example Steel E 0.21 1.64 2.34 0.01 0.002 0.03 0.001 B:0.002 Example Steel F 0.22 1.31 2.21 0.01 0.001 0.03 0.001 Cr: 0.20Example Steel G 0.20 1.38 2.14 0.01 0.001 0.03 0.002 Mo: 0.20 ExampleSteel H 0.19 1.84 2.01 0.01 0.001 0.03 0.002 Cu: 0.10 Example Steel I0.21 1.65 2.21 0.01 0.001 0.03 0.003 Ni: 0.10 Example Steel J 0.19 1.541.89 0.01 0.002 0.03 0.002 Ca: 0.0035 Example Steel K 0.22 1.45 2.220.01 0.002 0.03 0.002 REM: 0.0028 Example Steel L 0.11 1.50 2.34 0.010.002 0.03 0.002 — Comparative Steel M 0.21 0.34 2.48 0.01 0.002 0.020.003 — Comparative Steel N 0.22 2.12 1.21 0.01 0.002 0.03 0.003 —Comparative Steel O 0.19 0.88 3.01 0.02 0.002 0.04 0.002 — ComparativeSteel Remainder which is different from the constituent chemicalelements described above: Fe and inevitable impurities Underlinedportion: out of the range according to the present invention

TABLE 2 Annealing (First) Condition Annealing (Second) Condition FirstFirst Second Second Average Third Third Fourth Fourth Soaking HoldingSoaking Holding Heating Soaking Holding Soaking Holding Sample SteelTemperature Time CR1 (*1) Temperature Time Rate Temperature Time CR2(*1) Tq (*2) CR3 (*1) Temperature Time No. Grade ° C. sec ° C./s ° C.sec ° C./s ° C. sec ° C./s ° C. ° C./s ° C. sec 1 A 850 300 10 400 60010 800 600  5 400 880 250 600 2 B 860 600 10 400 300 10 800 600  5 400900 250 600 3 C 860 600 10 350 600 10 800 600  5 400 900 200 600 4 D 860300 10 350 600  5 820 500 10 400 800 250 600 5 E 850 300 10 400 300 10800 300 10 400 840 300 300 6 F 850 200 10 420 600 10 820 120 15 420 880250 300 7 G 850 120 10 400 300 10 790 300 10 400 800 250 300 8 H 860 30010 400 300 15 790 300 10 380 800 250 300 9 I 860 300 20 420 300 10 800300 10 400 890 250 300 10 J 850 120 10 400 300 10 800 300 10 400 880 250300 11 K 850 100  5 420 600 10 820 600 10 400 900 250 300 12 L 850 30010 400 600 10 800 300 10 400 880 250 600 13 A 750 120 10 380 600  5 820300 10 400 880 300 600 14 A 850  3 10 400 600 10 810 300  5 380 900 250600 15 A 850 300  1 400 600 10 800 300 10 400 900 250 600 16 A 850 30010 200 600 10 800 300 10 450 900 250 600 17 A 850 300 10 550 300 10 810300 10 400 880 250 600 18 A 850 300 10 400  10 10 810 300 10 400 880 250600 19 A 850 300 10 420 300  1 810 300 10 400 890 250 600 20 A 850 30010 400 600 10 720 300 10 400 900 250 600 21 A 850 300 10 400 600 10 810300  1 400 800 250 600 22 A 850 300 10 350 600 10 800 300 10 200 880 250600 23 A 860 300 10 400 300 10 800 300 10 600 890 250 600 24 A 850 30010 400 300 10 800 300 10 400 880 100 600 25 A 850 300 10 400 300 10 800600 10 400 900 550 600 26 A 850 300 10 400 300 10 800 300 10 400 800 250 10 27 L 850 300 10 400 300 10 800 300 10 400 900 250 300 28 M 850 30010 400 300 10 800 300 10 400 880 250 300 29 N 850 300 10 400 300 10 800300 10 450 880 250 600 30 O 850 300 10 400 300 10 820 300 10 400 900 250600 Underlined portion: out of the range according to the presentinvention (*1) CR1, CR2, and CR3 (° C./s): respectively the firstaverage cooling rate, the second average cooling rate, and the thirdaverage cooling rate in this order (*2) Tq (° C.): cooling stoptemperature of cooling performed at the second average cooling rate(cooling start temperature of cooling performed at the third averagecooling rate)

By taking a JIS No. 5 tensile test piece from the manufactured steelsheet so that a direction at a right angle to the rolling direction wasthe longitudinal direction (tensile direction) of the test piece, and byperforming a tensile test (JIS Z 2241 (1998)), yield strength (YS),tensile strength (TS), elongation (EL), and yield ratio (YR) weredetermined. A steel sheet having a tensile strength (TS) of 980 MPa ormore was judged as a high-strength steel sheet, a steel sheet having anelongation (EL) of 19% or more was judged as a steel sheet having a goodelongation (EL), and a steel sheet having a yield ratio (YR) of 66% orless was judged as a steel sheet having the desired low yield ratio(YR).

In addition, regarding hole expansion capability, in accordance with TheJapan Iron and Steel Federation Standard (JFS T 1001 (1996)), bypunching a hole having a diameter of 10 mmϕ in a sample with a clearanceof 12.5%, by setting the sample on a testing machine so that the burrwas on the die side, and by forming the sample with a conical punchhaving a point angle of 60° , hole expansion ratio (λ) was determined. Asteel sheet having a λ (%) of 30% or more was judged as a steel sheethaving good hole expansion capability.

Regarding steel sheet microstructure, by using a SEM (scanning electronmicroscope), a TEM (transmission electron microscope), and an FE-SEM(field-emission-type scanning electron microscope), steel sheetmicrostructure was observed in order to identify ferrite, retainedaustenite, tempered martensite, and other kinds of steelmicrostructures.

The volume fractions of ferrite and tempered martensite of the steelsheet were determined by polishing a cross section in the thicknessdirection parallel to the rolling direction of the steel sheet, by thenetching the polished cross section through the use of a 3%-nitalsolution, by observing the etched cross section through the use of a SEM(scanning electron microscope) at magnifications of 2000 times and 5000times, by determining the area fraction of each of the phases throughthe use of a point-counting method (in accordance with ASTM E562-83(1988)), and by defining the area fraction as the volume fraction.Regarding the average crystal grain diameters of ferrite, retainedaustenite, and tempered martensite, since it was possible to calculateeach area of the phases by inputting the steel sheet microstructurephotographs, in which the crystal grains of ferrite, retained austenite,and tempered martensite had been identified in advance, into Image-Proproduced by Media Cybernetics, Inc., by calculating circle-equivalentdiameters from the calculated areas, the average crystal grain diameterof each of the phases was defined as the average of the calculatedcircle-equivalent diameters.

The volume fraction of retained austenite was determined by polishingthe steel sheet in order to expose a surface located at ¼ of thethickness of the steel sheet and by determining the X-ray diffractionintensities of the surface. By determining the integrated intensities ofX-ray diffraction of. the (200) plane, (211) plane, and (220) plane ofthe ferrite of iron and the (200) plane, (220) plane, and (311) plane ofthe austenite of iron through the use of the Ka ray of Mo as a radiationsource with an acceleration voltage of 50 keV in X-ray diffractometry(apparatus: RINT-2200 produced by Rigaku Corporation), and by using thecalculating formula described in “X-ray Diffraction Handbook” publishedby Rigaku Corporation (2000), pp. 26 and 62-64, the volume fraction ofretained austenite was determined.

In addition, the number of grains of retained austenite were determinedby counting the number in the observation of a steel sheet photographtaken through the use of a SEM.

The determined steel sheet microstructure, tensile properties, and holeexpansion ratio (λ) are given in Table 3.

TABLE 3 Steel Sheet Microstructure Hole Number of Expansion FerriteRetained Austenite Tempered Martensite RA Grains in Tensile PropertyRatio Sample Volume Average Grain Volume Average Grain Volume AverageGrain Remainder an Area of YS TS EL YR λ No. Fraction/% Diameter/μmFraction/% Diameter/μm Fraction/% Diameter/μm Kind 1000 μm² MPa MPa % %% Note 1 46 3 7 1 42 1 TB 15 705 1082 22 65 37 Example 2 48 3 6 1 41 1TB 11 700 1065 21 66 36 Example 3 51 3 5 2 39 1 TB 13 688 1054 19 65 35Example 4 48 3 7 1 40 2 TB 12 643  990 22 65 36 Example 5 41 4 6 2 48 2TB 13 675 1025 20 66 35 Example 6 50 3 6 1 35 1 TB 15 661 1030 20 64 36Example 7 45 3 7 2 41 2 TB 16 664 1016 19 65 37 Example 8 48 4 5 1 43 1TB 14 664 1011 22 66 38 Example 9 50 3 6 1 39 2 TB 15 661 1003 23 66 36Example 10 50 3 6 1 40 2 TB 13 656 1041 22 63 40 Example 11 44 3 6 1 422 TB 12 684 1044 22 66 35 Example 12 41 3 7 1 46 1 TB 15 674 1032 21 6537 Example 13 38 4 6 3 49 4 TB  6 668 1033 19 65 28 Comparative Example14 44 3 5 3 38 5 TB  8 702 1029 19 68 25 Comparative Example 15 40 4 6 439 4 TB  6 721 1001 19 72 27 Comparative Example 16 44 4 5 3 41 5 TB  7709 1021 19 69 22 Comparative Example 17 46 4 6 3 38 3 TB  9 711 1009 1970 23 Comparative Example 18 47 4 5 4 38 3 TB  5 723 1046 20 69 20Comparative Example 19 48 6 5 2 40 3 TB 10 661 1002 19 66 22 ComparativeExample 20 71 5 6 2 17 2 TB  8 578  892 24 65 34 Comparative Example 2158 6 7 3 25 2 TB, P 11 645  964 23 67 28 Comparative Example 22 48 4 3 136 2 TB  4 681 1001 15 68 32 Comparative Example 23 42 3 2 2 50 4 TB  3691 1022 14 68 33 Comparative Example 24 44 3 2 2 41 2 TB  2 633 1033 1961 22 Comparative Example 25 43 4 2 2 40 2 TB  3 729 1039 18 70 33Comparative Example 26 43 3 4 1 40 2 TB  5 655 1030 19 64 19 ComparativeExample 27 69 4 3 2 27 2 TB  7 602  889 22 68 29 Comparative Example 2839 4 4 2 51 2 TB  3 655  989 18 66 30 Comparative Example 29 68 5 5 2 202 TB, P 10 595  876 24 68 35 Comparative Example 30 35 3 6 3 63 4 TB 11729 1088 16 67 22 Comparative Example Under lined portion: out of therange according to the present invention Microstructure: TB—temperedbainite, P—pearlite, RA—retained austenite

From the results given in Table 3, it is clarified that all the examplesof the present invention had a multi-phase microstructure includingferrite having an average crystal grain diameter of 5 μm or less in anamount of 30% to 55% in terms of volume fraction, retained austenitehaving an average crystal grain diameter of 2 μm or less in an amount of5% to 15% in terms of volume fraction, and tempered martensite having anaverage crystal grain diameter of 2 μm or less in an amount of 30% to60% in terms of volume fraction, and, as a result, had good formabilityrepresented by an elongation (EL) of 19% or more and a hole expansionratio (λ) of 30% or more while achieving a tensile strength of 980 MPaor more and a yield ratio (YR) of 66% or less.

On the other hand, in the case of No. 13 where the average crystal graindiameter of retained austenite was more than 2 μm, where the averagecrystal grain diameter of tempered martensite was more than 2 μm, andwhere the number of grains of retained austenite existing in an area of1000 gm² was less than 10, the hole expansion ratio (λ) was less than30%. In the case of Nos. 14 through 18 where the average crystal graindiameter of retained austenite was more than 2 μm, where the averagecrystal grain diameter of tempered martensite was more than 2 μm, andwhere the number of grains of retained austenite existing in an area of1000 μm² was less than 10, the yield ratio (YR) was more than 66%, andthe hole expansion ratio (λ) was less than 30%.

In addition, in the case of No. 19 where the average crystal graindiameter of ferrite was more than 5 μm and where the average crystalgrain diameter of tempered martensite was more than 2 μm, the holeexpansion ratio (λ) was less than 30%. In the case of No. 20 where thevolume fraction of ferrite was more than 55%, where the volume fractionof tempered martensite was less than 30%, and where the number of grainsof retained austenite existing in an area of 1000 μm² was less than 10,the tensile strength (TS) was less than 980 MPa.

In the case of No. 21 where the volume fraction of ferrite was more than55%, where the average crystal grain diameter of ferrite was more than 5μm, where the average crystal grain diameter of retained austenite wasmore than 2 μm, and where the volume fraction of tempered martensite wasless than 30%, the tensile strength (TS) was less than 980 MPa, theyield ratio (YR) was more than 66%, and the hole expansion ratio (λ) wasless than 30%. In the case of No. 22 where the volume fraction ofretained austenite was less than 5% and where the number of grains ofretained austenite existing in an area of 1000 μm² was less than 10, theelongation (EL) was less than 19%, and the yield ratio (YR) was morethan 66%.

In the case of No. 23 where the volume fraction of retained austenitewas less than 5%, where the average crystal grain diameter of temperedmartensite was more than 2 μm, and where the number of grains ofretained austenite existing in an area of 1000 μm² was less than 10, theelongation (EL) was less than 19%, and the yield ratio (YR) was morethan 66%.

In the case of Nos. 24 and 26 where the volume fraction of retainedaustenite was less than 5% and where the number of grains of retainedaustenite existing in an area of 1000 μm² was less than 10, the holeexpansion ratio (λ) was less than 30%. In the case of No. 25 where thevolume fraction of retained austenite was less than 5% and where thenumber of grains retained austenite existing in an area of 1000 μm² wasless than 10, the elongation (EL) was less than 19%, and the yield ratio(YR) was more than 66%.

In the case of No. 27 where the C content was less than 0.15 mass %,where the volume fraction of ferrite was more than 55%, where the volumefraction of retained austenite was less than 5%, where the volumefraction of tempered martensite was less than 30%, and where the numberof grains of retained austenite existing in an area of 1000 μm² was lessthan 10, the tensile strength (TS) was less than 980 MPa, the yieldratio (YR) was more than 66%, and the hole expansion ratio (λ) was lessthan 30%. In the case of No. 28 where the Si content was less than 1.0mass %, where the volume fraction of retained austenite was less than5%, and where the number of grains of retained austenite existing in anarea of 1000 μm² was less than 10, the elongation (EL) was less than19%.

In the case of No. 29 where the Mn content was less than 1.8 mass %,where the volume fraction of ferrite was more than 55%, and where thevolume fraction of tempered martensite was less than 30%, the tensilestrength (TS) was less than 980 MPa, and the yield ratio (YR) was morethan 66%. In the case of No. 30 where the Mn content was more than 2.5mass %, where the average crystal grain diameter of retained austenitewas more than 2 μm, where the volume fraction of tempered martensite wasmore than 60%, and where the average crystal grain diameter of temperedmartensite was more than 2 μm, the elongation (EL) was less than 19%,the yield ratio (YR) was more than 66%, and the hole expansion ratio (λ)was less than 30%.

1. A high-strength, cold-rolled steel sheet having a chemicalcomposition containing, by mass_%, C: 0.15% to 0.25%, Si: 1.0% to 2.0%,Mn: 1.8% to 2.5%, P: 0.10% or less, S: 0.010% or less, Al: 0.10% orless, N: 0.010% or less, and the balance being Fe and inevitableimpurities, and a multi-phase microstructure including ferrite having anaverage crystal grain diameter of 5μm or less in an amount of 30% to 55%in terms of volume fraction, retained austenite having an averagecrystal grain diameter of 2 μm or less in an amount of 5% to 15% interms of volume fraction, and tempered martensite having an averagecrystal grain diameter of 2 μpm or less in an amount of 30% to 60% interms of volume fraction, wherein the number of grains of the retainedaustenite existing in an area of 1000 μm² is 10 or more.
 2. Thehigh-strength, cold-rolled steel sheet according to claim 1, wherein thechemical composition further contains, by mass_%, one or more selectedfrom V: 0.10% or less, Nb: 0.10% or less, and Ti: 0.10% or less.
 3. Thehigh-strength, cold-rolled steel sheet according to claim 1 or 2,wherein the chemical composition further contains, by mass_%, B: 0.010%or less.
 4. The high-strength, cold-rolled steel sheet according toclaim 1, wherein the chemical composition further contains, by mass_%one or more selected from Cr: 0.50% or less, Mo: 0.50% or less, Cu:0.50% or less, Ni: 0.50% or less, Ca: 0.0050% or less, and REM: 0.0050%or less.
 5. A method for manufacturing the high-strength, cold-rolledsteel sheet according to claim 1, the method comprising, after havingperformed hot rolling and cold rolling on a steel slab to obtain acold-rolled steel sheet, performing continuous annealing on thecold-rolled steel sheet, the continuous annealing comprising: heatingthe cold-rolled steel sheet to a temperature of 850° C. or higher,holding the cold-rolled steel sheet at a first soaking temperature of850° C. or higher for 30 seconds or more, then cooling the cold-rolledsteel sheet from the first soaking temperature to a temperature of 320°C. to 500° C. at a first average cooling rate of 3° C./s or more,holding the cold-rolled steel sheet at a second soaking temperature of320° C. to 500° C. for 30 seconds or more, then cooling the cold-rolledsteel sheet to a temperature of 100° C. or lower, heating thecold-rolled steel sheet to a temperature of 750° C. or higher at anaverage heating rate of 3° C./s to 30° C./s, holding the cold-rolledsteel sheet at a third soaking temperature of 750° C. or higher for 30seconds or more, then cooling the cold-rolled steel sheet from the thirdsoaking temperature to a temperature of 350° C. to 500° C. at a secondaverage cooling rate of 3° C./s or more, cooling the cold-rolled steelsheet to a temperature of 100° C. or lower at a third average coolingrate of 100° C./s to 1000° C./s, heating the cold-rolled steel sheet toa temperature of 200° C. to 350° C., and then holding the cold-rolledsteel sheet at a fourth soaking temperature of 200° C. to 350° C. for120 seconds to 1200 seconds.
 6. The high-strength, cold-rolled steelsheet according to claim 2, wherein the chemical composition furthercontains, by mass %, B: 0.010% or less.
 7. The high-strength,cold-rolled steel sheet according to claim 2, wherein the chemicalcomposition further contains, by mass %, one or more selected from Cr:0.50% or less, Mo: 0.50% or less, Cu: 0.50% or less, Ni: 0.50% or less,Ca: 0.0050% or less, and REM: 0.0050% or less.
 8. The high-strength,cold-rolled steel sheet according to claim 3, wherein the chemicalcomposition further contains, by mass %, one or more selected from Cr:0.50% or less, Mo: 0.50% or less, Cu: 0.50% or less, Ni: 0.50% or less,Ca: 0.0050% or less, and REM: 0.0050% or less.
 9. The high-strength,cold-rolled steel sheet according to claim 6, wherein the chemicalcomposition further contains, by mass %, one or more selected from Cr:0.50% or less, Mo: 0.50% or less, Cu: 0.50% or less, Ni: 0.50% or less,Ca: 0.0050% or less, and REM: 0.0050% or less.
 10. A method formanufacturing the high-strength, cold-rolled steel sheet according toclaim 2, the method comprising, after having performed hot rolling andcold rolling on a steel slab to obtain a cold-rolled steel sheet,performing continuous annealing on the cold-rolled steel sheet, thecontinuous annealing comprising: heating the cold-rolled steel sheet toa temperature of 850° C. or higher, holding the cold-rolled steel sheetat a first soaking temperature of 850° C. or higher for 30 seconds ormore, then cooling the cold-rolled steel sheet from the first soakingtemperature to a temperature of 320° C. to 500° C. at a first averagecooling rate of 3° C./s or more, holding the cold-rolled steel sheet ata second soaking temperature of 320° C. to 500° C. for 30 seconds ormore, then cooling the cold-rolled steel sheet to a temperature of 100°C. or lower, heating the cold-rolled steel sheet to a temperature of750° C. or higher at an average heating rate of 3° C./s to 30° C./s,holding the cold-rolled steel sheet at a third soaking temperature of750° C. or higher for 30 seconds or more, then cooling the cold-rolledsteel sheet from the third soaking temperature to a temperature of 350°C. to 500° C. at a second average cooling rate of 3° C./s or more,cooling the cold-rolled steel sheet to a temperature of 100° C. or lowerat a third average cooling rate of 100° C./s to 1000° C./s, heating thecold-rolled steel sheet to a temperature of 200° C. to 350° C., and thenholding the cold-rolled steel sheet at a fourth soaking temperature of200° C. to 350° C. for 120 seconds to 1200 seconds.
 11. A method formanufacturing the high-strength, cold-rolled steel sheet according toclaim 3, the method comprising, after having performed hot rolling andcold rolling on a steel slab to obtain a cold-rolled steel sheet,performing continuous annealing on the cold-rolled steel sheet, thecontinuous annealing comprising: heating the cold-rolled steel sheet toa temperature of 850° C. or higher, holding the cold-rolled steel sheetat a first soaking temperature of 850° C. or higher for 30 seconds ormore, then cooling the cold-rolled steel sheet from the first soakingtemperature to a temperature of 320° C. to 500° C. at a first averagecooling rate of 3° C./s or more, holding the cold-rolled steel sheet ata second soaking temperature of 320° C. to 500° C. for 30 seconds ormore, then cooling the cold-rolled steel sheet to a temperature of 100°C. or lower, heating the cold-rolled steel sheet to a temperature of750° C. or higher at an average heating rate of 3° C./s to 30° C./s,holding the cold-rolled steel sheet at a third soaking temperature of750° C. or higher for 30 seconds or more, then cooling the cold-rolledsteel sheet from the third soaking temperature to a temperature of 350°C. to 500° C. at a second average cooling rate of 3° C./s or more,cooling the cold-rolled steel sheet to a temperature of 100° C. or lowerat a third average cooling rate of 100° C./s to 1000° C./s, heating thecold-rolled steel sheet to a temperature of 200° C. to 350° C., and thenholding the cold-rolled steel sheet at a fourth soaking temperature of200° C. to 350° C. for 120 seconds to 1200 seconds.
 12. A method formanufacturing the high-strength, cold-rolled steel sheet according toclaim 6, the method comprising, after having performed hot rolling andcold rolling on a steel slab to obtain a cold-rolled steel sheet,performing continuous annealing on the cold-rolled steel sheet, thecontinuous annealing comprising: heating the cold-rolled steel sheet toa temperature of 850° C. or higher, holding the cold-rolled steel sheetat a first soaking temperature of 850° C. or higher for 30 seconds ormore, then cooling the cold-rolled steel sheet from the first soakingtemperature to a temperature of 320° C. to 500° C. at a first averagecooling rate of 3° C./s or more, holding the cold-rolled steel sheet ata second soaking temperature of 320° C. to 500° C. for 30 seconds ormore, then cooling the cold-rolled steel sheet to a temperature of 100°C. or lower, heating the cold-rolled steel sheet to a temperature of750° C. or higher at an average heating rate of 3° C./s to 30° C./s,holding the cold-rolled steel sheet at a third soaking temperature of750° C. or higher for 30 seconds or more, then cooling the cold-rolledsteel sheet from the third soaking temperature to a temperature of 350°C. to 500° C. at a second average cooling rate of 3° C./s or more,cooling the cold-rolled steel sheet to a temperature of 100° C. or lowerat a third average cooling rate of 100° C./s to 1000° C./s, heating thecold-rolled steel sheet to a temperature of 200° C. to 350° C., and thenholding the cold-rolled steel sheet at a fourth soaking temperature of200° C. to 350° C. for 120 seconds to 1200 seconds.
 13. A method formanufacturing the high-strength, cold-rolled steel sheet according toclaim 4, the method comprising, after having performed hot rolling andcold rolling on a steel slab to obtain a cold-rolled steel sheet,performing continuous annealing on the cold-rolled steel sheet, thecontinuous annealing comprising: heating the cold-rolled steel sheet toa temperature of 850° C. or higher, holding the cold-rolled steel sheetat a first soaking temperature of 850° C. or higher for 30 seconds ormore, then cooling the cold-rolled steel sheet from the first soakingtemperature to a temperature of 320° C. to 500° C. at a first averagecooling rate of 3° C./s or more, holding the cold-rolled steel sheet ata second soaking temperature of 320° C. to 500° C. for 30 seconds ormore, then cooling the cold-rolled steel sheet to a temperature of 100°C. or lower, heating the cold-rolled steel sheet to a temperature of750° C. or higher at an average heating rate of 3° C./s to 30° C./s,holding the cold-rolled steel sheet at a third soaking temperature of750° C. or higher for 30 seconds or more, then cooling the cold-rolledsteel sheet from the third soaking temperature to a temperature of 350°C. to 500° C. at a second average cooling rate of 3° C./s or more,cooling the cold-rolled steel sheet to a temperature of 100° C. or lowerat a third average cooling rate of 100° C./s to 1000° C./s, heating thecold-rolled steel sheet to a temperature of 200° C. to 350° C., and thenholding the cold-rolled steel sheet at a fourth soaking temperature of200° C. to 350° C. for 120 seconds to 1200 seconds.
 14. A method formanufacturing the high-strength, cold-rolled steel sheet according toclaim 7, the method comprising, after having performed hot rolling andcold rolling on a steel slab to obtain a cold-rolled steel sheet,performing continuous annealing on the cold-rolled steel sheet, thecontinuous annealing comprising: heating the cold-rolled steel sheet toa temperature of 850° C. or higher, holding the cold-rolled steel sheetat a first soaking temperature of 850° C. or higher for 30 seconds ormore, then cooling the cold-rolled steel sheet from the first soakingtemperature to a temperature of 320° C. to 500° C. at a first averagecooling rate of 3° C./s or more, holding the cold-rolled steel sheet ata second soaking temperature of 320° C. to 500° C. for 30 seconds ormore, then cooling the cold-rolled steel sheet to a temperature of 100°C. or lower, heating the cold-rolled steel sheet to a temperature of750° C. or higher at an average heating rate of 3° C./s to 30° C./s,holding the cold-rolled steel sheet at a third soaking temperature of750° C. or higher for 30 seconds or more, then cooling the cold-rolledsteel sheet from the third soaking temperature to a temperature of 350°C. to 500° C. at a second average cooling rate of 3° C./s or more,cooling the cold-rolled steel sheet to a temperature of 100° C. or lowerat a third average cooling rate of 100° C./s to 1000° C./s, heating thecold-rolled steel sheet to a temperature of 200° C. to 350° C., and thenholding the cold-rolled steel sheet at a fourth soaking temperature of200° C. to 350° C. for 120 seconds to 1200 seconds.
 15. A method formanufacturing the high-strength, cold-rolled steel sheet according toclaim 8, the method comprising, after having performed hot rolling andcold rolling on a steel slab to obtain a cold-rolled steel sheet,performing continuous annealing on the cold-rolled steel sheet, thecontinuous annealing comprising: heating the cold-rolled steel sheet toa temperature of 850° C. or higher, holding the cold-rolled steel sheetat a first soaking temperature of 850° C. or higher for 30 seconds ormore, then cooling the cold-rolled steel sheet from the first soakingtemperature to a temperature of 320° C. to 500° C. at a first averagecooling rate of 3° C./s or more, holding the cold-rolled steel sheet ata second soaking temperature of 320° C. to 500° C. for 30 seconds ormore, then cooling the cold-rolled steel sheet to a temperature of 100°C. or lower, heating the cold-rolled steel sheet to a temperature of750° C. or higher at an average heating rate of 3° C./s to 30° C./s,holding the cold-rolled steel sheet at a third soaking temperature of750° C. or higher for 30 seconds or more, then cooling the cold-rolledsteel sheet from the third soaking temperature to a temperature of 350°C. to 500° C. at a second average cooling rate of 3° C./s or more,cooling the cold-rolled steel sheet to a temperature of 100° C. or lowerat a third average cooling rate of 100° C./s to 1000° C./s, heating thecold-rolled steel sheet to a temperature of 200° C. to 350° C., and thenholding the cold-rolled steel sheet at a fourth soaking temperature of200° C. to 350° C. for 120 seconds to 1200 seconds.
 16. A method formanufacturing the high-strength, cold-rolled steel sheet according toclaim 9, the method comprising, after having performed hot rolling andcold rolling on a steel slab to obtain a cold-rolled steel sheet,performing continuous annealing on the cold-rolled steel sheet, thecontinuous annealing comprising: heating the cold-rolled steel sheet toa temperature of 850° C. or higher, holding the cold-rolled steel sheetat a first soaking temperature of 850° C. or higher for 30 seconds ormore, then cooling the cold-rolled steel sheet from the first soakingtemperature to a temperature of 320° C. to 500° C. at a first averagecooling rate of 3° C./s or more, holding the cold-rolled steel sheet ata second soaking temperature of 320° C. to 500° C. for 30 seconds ormore, then cooling the cold-rolled steel sheet to a temperature of 100°C. or lower, heating the cold-rolled steel sheet to a temperature of750° C. or higher at an average heating rate of 3° C./s to 30° C./s,holding the cold-rolled steel sheet at a third soaking temperature of750° C. or higher for 30 seconds or more, then cooling the cold-rolledsteel sheet from the third soaking temperature to a temperature of 350°C. to 500° C. at a second average cooling rate of 3° C./s or more,cooling the cold-rolled steel sheet to a temperature of 100° C. or lowerat a third average cooling rate of 100° C./s to 1000° C./s, heating thecold-rolled steel sheet to a temperature of 200° C. to 350° C., and thenholding the cold-rolled steel sheet at a fourth soaking temperature of200° C. to 350° C. for 120 seconds to 1200 seconds.